1. Field of the Invention
This invention pertains to methods and apparatuses for increasing the vertical and, more importantly, the lateral load-carrying capacity of a pile or pile-like supports in soil having an upper, seasonally frozen region and a lower, permanently frozen region. The invention is especially useful with refrigerating-type thermal piles which have the capability of removing heat from the permanently frozen region of the soil to increase the load-carrying capacity of the surrounding soil.
2. Description of the Prior Art
Thermal piles for use in increasing load-carrying capacity have been used heretofore. One such pile is described in my earlier U.S. Pat. No. 3,217,791. The lowering of the temperature of the permanently frozen region of permafrost soil increases the vertical and lateral load-carrying capacity of the soil surrounding the pile.
The common technique for meeting the load-carrying capacity of the piles used in a large building is to increase the size or diameter of the piles at the numerous locations around the building. It is expensive to manufacture and transport large thermal piles due to their weight and size. It is also expensive due to the fact that a thermal pile unit is essentially a pressure vessel which must be manufactured according to strict specifications. Large pressure vessels are extremely expensive. The lateral load-carrying capacity of a large-diameter pile is frequently less than is required for a pile designed to have a certain vertical load-carrying capacity. Consequently, large piles are either over-designed, with excess vertical load-carrying capacity, or cross-bracing between piles in holes spaced around the building is provided between adjacent piles to obtain the necessary lateral load-carrying capacity. Both solutions, however, are expensive.
Stepped piles, having larger diameters at their upper ends than at their lower ends, are known. A stepped pile is useful in permafrost having a lower, permanently frozen region since the larger stepped diameter can be terminated below the upper surface of the permanently frozen region and not extend the full length of the pile without substantially reducing the lateral load-carrying capacity of the pile. By stepping the pile, the remaining lower portion of the pile can be built of considerably less material, providing a substantial cost savings. The reason for this characteristic of a stepped pile in providing substantially the same lateral load-carrying capacity as a continuous larger diameter pile in frozen soil is that the point of maximum inflection along the length of the pile occurs on the pile approximately at the top surface of the permanently frozen region of the soil. That is, the stress distribution for a top laterally loaded pile is at a minimum at the point of loading and increases substantially uniformly along the length of the pile until it reaches the general area of the top surface of the permanently frozen region of the soil. Below the top surface of the permanently frozen region of the soil, the stress in the pile is reduced rapidly along the length of the pile until a point is reached along the length of the pile where little or zero stress is applied to the pile. Since the lowermost part of the pile provides little lateral load-carrying capacity, the total lateral load-carrying capacity of the pile can be increased merely by increasing the portion of the pile extending upwardly from within the permanently frozen region of the soil. While stepped pile and the advantages of stepped pile for use in permafrost are thus generally known, the maximum cost benefits have never been obtained.